This invention relates to a telephone circuit including a loudspeaker, and more particularly to a telephone circuit wherein the magnitude of the loss inserted in the telephone circuit for the purpose of preventing howling of a loudspeaking telephone set is automatically varied in accordance with the condition of use of the telephone set.
As is well known in the art, a prior art loudspeaking telephone set comprises a telephone circuit including a microphone, a transmitting amplifier, an anti-sidetone circuit, a receiving amplifier and a loudspeaker; and a voice switch with its one side connected with the microphone and the loudspeaker of the telephone circuit and the other side connected with an anti-sidetone circuit. The purpose of the voice switch connected as described above is to prevent howling caused by the acoustic coupling between the microphone and the loudspeaker. More particularly, due to the presence of an amount of the acoustic transmission .alpha. between the microphone and the loudspeaker systems, and an amount of the transmission .beta. on the other side through the anti-sidetone circuit, when the gain of the loop comprised by these .alpha. and .beta. transmission exceeds a unity, howling occurs. For this reason, the voice switch is used such that it inserts a loss in the receiving circuit by detecting the transmitting signal sent by a user (this condition of the circuit is termed a "transmitting condition") and inserts a loss in the transmitting circuit by detecting a received signal at the time of reception (this condition of the circuit is termed a "receiving condition"). In this manner, by the insertion of a loss L[dB] in the speech circuit the gain of said loop is maintained below 0[dB], thereby preventing the howling. As described above, the value of the loss L[dB] to be inserted in the speech circuit is determined by the magnitude of the sum (.alpha.+.beta.) [dB]. The magnitude of .alpha. increases as the distance between the loudspeakers and the microphone decreases whereas the value of .beta. increases as the sidetone balance is lost. For this reason, when designing a loudspeaking telephone set the value of the loss L[dB] inserted by the voice switch has been determined such that the loss L[dB] inserted is larger than the sum of the maximum values of .alpha. and .beta. which may occur during the use of the telephone set so as to prevent the howling in any operating condition.
As an example, the numerical data where the line loss connected with the telephone set is 10[dB] will be described hereinunder. Suppose now that the distance between the loudspeaker and the microphone is equal to 50 cm., and that the impedance of the line connected to the telephone set is equal to 600 ohms, the value of the required loss will be approximately L = 10[dB]. However, when the user approaches the loudspeaker to the microphone to a distance of 10 cm., the acoustic coupling between the loudspeaker and the microphone will be increased by about 20 log (50/10) .congruent. 14[dB]. In other words, the value of .alpha. increases by 14[dB]. Moreover, when telephone sets are interconnected through a telephone exchange, the impedance of the telephone line termination is momentarily opened so that the line impedance connected to a telephone set becomes much larger than the inherent line impedance. Under these conditions, the anti-sidetone characteristic decreases by 6[dB] comparing with a 600 ohm line impedance thereby increasing the value of .beta. by 6[dB]. Namely, to prevent howling when the line termination is opened, with a distance of 10 cm. between the microphone and the loudspeaker, it is necessary to increase the loss inserted by the voice switch by an amount corresponding to the increase in the values of .alpha. and .beta. than the aforementioned case in which L = 10[dB]. Thus, the required loss amounts to L = 30[dB]. In this manner, the prior art loudspeaking telephone set is designed such that a loss of L = 30[dB] is inserted by the voice switch where the line loss is equal to 10[dB]. Since the value of the "loss L" is determined by considering the worst condition of the use of the loudspeaking telephone set from the viewpoint of howling, in most cases, a loss larger than that required is inserted, so that the speech signals during transmission and reception are interrupted by the voice switch thus making it difficult to communicate with a loudspeaking telephone set.
Furthermore, in a conventional loudspeaking telephone set, since the voice switch inserts a loss in either one of the transmitting and receiving channels during talking, when two talkers speak simultaneously either one of the signals is attenuated by the inserted loss with the result that there is a case in which it would be impossible to listen to the signal if the inserted loss were too large. It has also been recognized that since in order to switch the loss in the transmitting and receiving channels it is necessary to compare the levels of the transmitted and received signals and since the switching of the loss accompanies a time lag, when one of the talkers offers while the other talker speaks or immediately after completion of his speech, the initial syllables of the speech burst of the signal of the offering party will be interrupted due to the loss inserted by the voice switch thus obscuring the speech. Further, in an environment in which room noise is excessive or a large line noise exists, the loss inserted by the voice switch will be kept in the transmitting or receiving channel by the noise thereby disenabling mutual communication.